1. Field of the Disclosure
The present disclosure relates to a self-refresh period measurement circuit of a semiconductor device, and more particularly to a self-refresh period measurement circuit of a semiconductor device which is capable of measuring a more accurate self-refresh period of the semiconductor device.
2. Description of the Related Art
One of the most important matters in mobile products such as mobile phones, notebook computers, etc., is how long the products can successfully be operated with given batteries. In this regard, it is very important in mobile dynamic random access memories (DRAMs) installed in such products to reduce self-refresh current that is generated in a standby state of the DRAMs.
Functions such as a Partial Array Self Refresh (PASR) mode, Temperature Compensated Self Refresh (TCSR) mode and Deep Power Down (DPD) mode are generally installed in such mobile products to reduce self-refresh current of the products so as to reduce power consumption thereof. Among these, the PASR mode and TCSR mode are programmed and used by the user to utilize Extended Mode Register Set (EMRS) feature.
In general, the data preservation time of a device at low temperature is longer than that at high temperature. In this regard, a TCSR circuit can reduce power consumption of a DRAM by varying a self-refresh period of the DRAM with temperature in such a manner as to shorten the self-refresh period when the DRAM is used at high temperature and lengthen the self-refresh period when the DRAM is used at low temperature. In particular, in an auto TCSR circuit, a temperature setting is not performed by the user, but the temperature of a chip is sensed and the period of an oscillation signal for a refresh operation is automatically adjusted according to the sensed temperature.
In order to automatically adjust a self-refresh period of a semiconductor device such as a mobile DRAM or the like according to temperature to reduce current consumption of the semiconductor device as stated above, it is very important to measure how long the self-refresh period actually is. However, a conventional self-refresh period measurement circuit has a disadvantage in that operation characteristics thereof are unstable when measuring a self-refresh period, as will hereinafter be described in detail with reference to FIG. 1.
FIG. 1 is a timing diagram showing the waveforms of respective signals in a conventional self-refresh period measurement circuit. Here, a control signal TM_REF is a test mode signal that enables a test mode for measuring a self-refresh period. A control signal OSC_MEAS_ON is a pulse that is enabled high in level when a semiconductor device enters a self-refresh mode, to indicate that the semiconductor device has entered the self-refresh mode. The semiconductor device performs a first cycle of a self-refresh operation (also referred to hereinafter as a “first self-refresh cycle”) at a time t2 that the control signal OSC_MEAS_ON is enabled. A control signal OSC is an oscillation signal that is periodically enabled after the semiconductor device enters the self-refresh mode, to allow the self-refresh operation to be periodically performed. The semiconductor device performs the self-refresh operation each time the control signal OSC makes a high level transition. The control signal OSC allows the self-refresh operation to be performed periodically beginning with a second cycle.
The self-refresh period is conventionally measured by measuring a period corresponding to the first cycle of the self-refresh operation, namely, a period between the enable time t2 of the control signal OSC_MEAS_ON and an enable time t3 of the oscillation signal OSC, as shown in FIG. 1. That is, the conventional self-refresh period measurement circuit generates a refresh period output signal REF_OSC that is enabled for the period from the time t2 that the control signal OSC_MEAS_ON is enabled to the time t3 that the oscillation signal OSC is enabled for the first time. The conventional circuit then measures the self-refresh period by counting the number of specific signals, such as clocks with a certain period, strobed for the enable period (from t2 to t3).
However, an oscillator of an auto TCSR circuit is liable to operate unstably in the first self-refresh cycle that is performed simultaneously with the self-refresh mode entry. For this reason, it is unreasonable to recognize that the measurement result of the period of the first self-refresh cycle represents an accurate self-refresh period. Nevertheless, the conventional self-refresh period measurement circuit takes, as the self-refresh period, the period in which the refresh period output signal REF_OSC is enabled, namely, the period in which the first self-refresh cycle is performed, as described above, so that it cannot measure an accurate period of the self-refresh operation, thereby causing the auto TCSR circuit not to perform the self-refresh operation appropriately for the chip temperature.